Conventionally, devices used for a purpose of detecting an amount of change of infrared light emitted from a human body in a predetermined detection area and controlling apparatuses such as lighting apparatuses are proposed as an optical detection device (e.g., Japanese Patent Publication Nos. 3090336 and 3090337).
Optical detection devices described in the aforementioned two documents each include a multi-segment lens configured by combining a plurality of lenses having substantially the same focal position on a single plane, and an infrared detecting element that is a light receiving element arranged on the aforementioned focal position. The aforementioned two documents describe that 4*15 detection beams are created, in a case where the infrared detecting element configured by arranging four light receiving units (device elements) is employed, and the multi-segment lens configured by arranging fifteen lenses in five columns and three rows is employed.
In each lens of the multi-segment lens, a first surface is a plane surface, a second surface is a hyperboloid having a principal axis oblique to a normal line of the first surface. Specifically, as shown in FIG. 19, in a lens 101, a rotation axis C of a hyperboloid 120 that is the second surface is inclined so that the rotation axis C and a normal line H of a plane surface 110 that is the first surface form an angle θ.
Here, assuming that L denotes a distance between an apex O of the lenses 101 and a center of an infrared detecting element (not shown), and f denotes a focal distance of the lens 101, an angle δ formed by incident light D reaching a focal point F through the apex O of the lens with respect to the normal line H is obtained according to the following Formula:[Math. 1]δ=arctan(L/f)  Formula (1).
Assuming that n denotes a refractive index of the lens 101, the following Formula is obtained:[Math. 2]n·sin θ=sin δ  Formula (2).
Therefore, the angle θ is as follows:[Math. 3]θ=arcsin(sin δ/n)  Formula (3).
The angle θ is determined according to Formula (3).
The lens 101 is provided as a light collection optical system, and the incident light D reaching the focal point F through the apex O forms the angle δ with respect to the normal line H, and is aplanatically collected on the focal point F. In this lens 101, when the angle θ is increased, the angle δ formed by the incident light D aplanatically collected on the focal point F and the normal line H is also increased.
The aforementioned two documents describe that polyethylene is employed as a material of the multi-segment lens, and the multi-segment lens is produced by injection molding.
However, even when the lens 101 formed by polyethylene has a thickness of 1 mm, transmittance of infrared light with a wavelength of around 10 μm, which is vertically incident upon the plane surface 110 of the lens 101, is 40%. In addition, the larger the thickness is, the lower the transmittance is. A light pass length of the incident light D incident from a direction not perpendicular to the plane surface 110 of the lens 101 may be longer than a maximum thickness of the lens 101, and the transmittance thereof may become too lower. Furthermore, in the lens 101 formed by polyethylene, in a case where the change in the thickness is large, sink marks occur on the surface of the lens 101 due to cooling of the injection molding, shrinkage unevenness caused during a solidification process, or the like. Therefore, appearance of the lens 101 is damaged.
So, in order to suppress the reduction in the transmittance and the occurrence of the sink marks, the aforementioned two documents describe that a minimum thickness of the lens 101 is set to 0.3 mm that is a minimum value allowable in the light of liquidity of polyethylene in injection molding, and difference between the maximum and minimum thicknesses of the lens 101, which has an influence on securement of a lens area (effective lens area), is set to 0.5 mm that is a minimum value necessary for securing a predetermined lens area depending on the intended use of an optical detection device (for sink light), so that the maximum thickness of the lens 101 is kept to be 1 mm or less.
Japanese Examined Patent Publication No. 7-36041 proposes that a condenser lens 401 is a Fresnel lens, and a rotation axis C shared by hyperboloids 421, 422 and 423 of a second surface is oblique to a plane surface 410 of a first surface in order to suppress occurrence of an off-axis aberration, as shown in FIG. 20. In this case, the hyperboloids 421, 422 and 423 configure respective lens surfaces.
The aforementioned document describes that in the Fresnel lens 401 in FIG. 20, an angle can be formed between a parallel light beam aplanatically collected on a focal point and a normal line N of the plane surface 410 according to an angle formed by the rotation axis C shared by the hyperboloids 421, 422 and 423 and the plane surface 410. Therefore, in the Fresnel lens 401 in FIG. 20, the occurrence of the off-axis aberration can be suppressed, and light beams from a direction oblique to the normal line N of the plane surface 410 can be effectively collected.
However, in the Fresnel lens 401 configured such that the rotation axis C of the hyperboloids 421, 422 and 423 configuring an emission surface is oblique to the normal line N of the plane surface 410 that is an incident surface, the hyperboloids 421, 422 and 423 are not rotationally symmetric with respect to the normal line N of the plane surface 410. Therefore, the Fresnel lens 401 or a metal mold for the Fresnel lens 401 is difficult to be produced by rotary forming with a lathe or the like.
So, when the Fresnel lens 401 or the metal mold for the Fresnel lens 401 is produced, it is necessary to use a multiaxis control processing machine and form the hyperboloids 421, 422 and 423 or respective curved surfaces by cutting at minute pitches while only a blade edge of a sharp cutting tool (tool) 430 with a nose radius (also referred to as a corner radius) of a few micro-meters is brought into point contact with a workpiece 440, as shown in FIG. 21. The workpiece 440 is a base material for directly forming the Fresnel lens 401, or a base material for forming the metal mold. Therefore, the processing time for producing the aforementioned Fresnel lens 401 or metal mold for the Fresnel lens 401 is increased, and then the cost of the Fresnel lens 401 is increased.
On the other hand, in a case where the cross-sectional shape of each lens surface in the cross-sectional shape including the normal line of the plane surface that is the incident surface of the Fresnel lens is linear, the lens surfaces or the curved surfaces corresponding to the lens surfaces can be formed by cutting while the cutting tool 430 is inclined with respect to the workpiece 440 so as to bring a side surface of a blade into line contact with the workpiece 440, as shown in FIG. 22, thus enabling significant reduction of the processing time. Here, in a Fresnel lens in which the shape of each lens surface in an emission surface is rotationally symmetric by employing a normal line of the incident surface as a rotation axis, it is known that each lens surface is approximated by a side surface of a frustum of cone, thereby enabling the cross-sectional shape of each lens surface to become linear (U.S. Pat. No. 4,787,722).
In the Fresnel lens 401 disclosed in the aforementioned Japanese Examined Patent Publication No. 7-36041 and the Fresnel lens disclosed in the U.S. Pat. No. 4,787,722, an intended light beam is infrared light, and these two documents disclose that polyethylene is used as a lens material.
Incidentally, the present inventors have conceived that, in the case where an apparatus is equipped with an optical detection device, since a multi-segment lens configures a part of the appearance of the apparatus, a surface, upon which infrared light is incident, of each lens of the multi-segment lens is formed to a plane surface or a curved surface with small curvature, in order not to damage the design of the apparatus. In a case where the apparatus equipped with the optical detection device is an alarm sensor, the present inventors have conceived that it is necessary to provide the optical detection device so that a suspicious person cannot find the presence of the optical detection device or a detection area of the alarm sensor. Furthermore, the present inventors have conceived that in a case where examples of the apparatus equipped with the optical detection device include an apparatus, in which a distance between a person and an optical detection device is relatively short, such as a television or a display of a personal computer, and an apparatus such as an alarm sensor, the appearance of the multi-segment lens is important, and lens patterns preferably cannot be visually recognized even when looking into the apparatus from a relatively close (e.g., about 30 cm) place. So, in the optical detection device described above, it is conceived that the difference between the maximum and minimum thicknesses of each lens 101 of the multi-segment lens is set to a value smaller than 0.5 mm described above, for example. However, while this can make it difficult to visually recognize the lens patterns, the predetermined lens area cannot be secured, and consequently, sensitivity is reduced.
Thus, an option of employing the Fresnel lens as a lens capable of making it difficult to visually recognize the lens patterns formed on the second surface is offered. However, in the Fresnel lens in which the shape of each lens surface in an emission surface is rotationally symmetric by employing the normal line of the incident surface as the rotation axis, in the lens surface being approximated by the side surface of the frustum of cone, an off-axis aberration occurs in a case of utilizing incident light (e.g., infrared light) obliquely incident upon the first surface from the outside world.